46386926 suspension geometry

8/13/2019 46386926 Suspension Geometry

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Suspension Geometry

Basic ConceptRide

Basically, suspensions are employed to deal with hump in road surface, in other words,enhancing ride comfort. When a car rides over a hump, the springs are compressed,store the energy thus provide shock absorption. The energy will be released quickly whenthe springs bounce back. Dampers are employed to smooth and slow down the bouncemotion, this is called "Damping". Without dampers, the car will bounce up and downseverely and quickly, this is perceived as uncomfortable. tudy found that ride isperceived as comfortable by human when the bouncing frequency is ! to !. #$. %f ite&peeds ' #$, most people feel the ride harsh. Therefore ride quality is mostly controlledby the selection of suitable springs and dampers. Handling

%n order to achieve ride comfort, we create suspensions and let the wheels movable withrespect to the car body. %nevitably, this create many many problems in handling. Whenthe car is turning quickly into a bend, centrifugal force will roll the car body. Body rollleads to the weight transfer towards the outside wheels, it also changes the suspensionsgeometry which changes the camber angles of wheels. (hange of camber accompanieswith weight transfer result in unwanted understeering or oversteering. %f brakes areapplied in the bend, castor angles will also be changed, that may further deterioratedundersteering ) oversteering or even introduces torque steer. *Don+t understand -o

problem, you will have a clearer view in the following paragraphs Camber - Decisive to understeering and oversteering

This is a very important concept. We must learn this before going on our study. /s shown in below, if a wheel is not perpendicular to the road, then it is cambered. %f itleans towards the center of the car, then it is negative cambered. %f it leans outwards tothe car, it is positive cambered *as shown in the following picture.

When a wheel haspositve camber, due to

the elasticity of tyres, thewheel will be reshaped tosomething like the baseof a cone. %t will have atendency to rotate aboutthe peak of the cone, asshown in the picture.-ow, you will see the

wheel tries to steer away from the center of the car.



%f both the right and left wheels are positive cambered *that means they lean towardsopposite directions, the steering tendency will be cancelled so that the car remainsrunning in straight line. %f the car is turning into a corner, body roll puts more weight onthe outside wheels than the inside wheels, that means the outside wheel+s steeringtendency will have more influence to the car. /s the positive0cambered outside wheeltries to steer the car to the outside of the corner, the car will be understeered. 1n the contrary, if both wheels are negative cambered, the car will oversteer. A Good Suspension must :

! 2rovide independent shock absorption to individual wheels. That means, when onewheel rides over a hump, the shock will not be transferred to other wheels. ' #as adequate body roll. 3&cessive body roll leads to too much weight transfer thusinfluence the steering response. %t is not comfortable too. 4estrict body roll to minimalmay create uncomfortable feeling because of e&cessive g0force. 5oreover, body rollcould provide information to the driver, telling him the state of cornering and whether thecar has reached its limit. (ompletely eliminate body roll is not at all good. 6 #as a good geometry such that wheel cambers remain unchanged in all conditions,that is, acceleration, braking, cornering, load and bumps. Body roll suppression usually conflict with ride comfort, because the former requiresstiffer spring and dampers while the latter vice versa. -evertheless, clever suspensiongeometry may improve body roll without altering the ride. #ere in below we are going todiscuss the most popular kinds of suspension geometry.

Non-independentsuspension

Live / Dead axle

7ntil the late 89s, most cars still used thissimple non0independent suspensions,especially at the rear a&le. Basically, it is a

rigid a&le fi&ed between left and rightwheels. The car body is suspended by leafsprings or coil springs on the a&le ) wheelsunit.

/s you can see, the wheels are notindependent. When one wheel rides on ahump, the shock will be transferred toanother wheel. Besides, both wheels will be

cambered, thus non0neutral steering is inevitable.



%f the a&le is also the driving a&le, it is called Live Axle. :ive a&le is very heavy. %tconsists of the final drive ) differential, drive shafts and a strong tube enclosing all thesethings. ince the whole a&le is rigidly fi&ed to the wheels instead of suspended bysprings, the so0called Unsprung Weigt is very high. What is the result of high unsprung weight /ssuming a live a&le meets a hump and

";ump" quickly upward, the more weight it has, the more momentum it gains *becausemomentum < the product of mass and velocity. That means the more momentum thesprings have to deal with. 1f course, springs cannot absorb all the momentum, soeventually part of the latter will be transferred to the car body in the form of shock.Therefore live a&le is never good at ride quality. %f the a&le is not the driving a&le, it is called Dead Axle. Without the driving mechanismincorporated, dead a&le has much less unsprung weight, so its ride quality is better than:ive /&le. /nyway, comparing them is useless 0 it does not make sense for a car to usedead a&le in the non0drive wheels while using a suspension advancer than live a&le in thedriving wheels. %ndependent 5ac2herson suspension is rather easy and cost0effectivelyto replace the non0drive dead a&le. %f a car maker cannot afford such little additional cost,it must not willing to employ advancer suspensions in the driving wheels also. :ive ) Dead /&le have another disadvantage 0 body roll is not sufficiently suppressed.prings are the only element which control the body roll, however, stiffen the springs willinevitably deteriorate ride quality. 5oreover, if coil springs are used, lateral force due tocornering will lead to transverse movement of the car body, thus result in weight transferand affect steering response. The popular solution was to add some control arms between the car body and the a&le,such as 2anhard rod and Watt link. #owever, they are out of our scope.

/dvantage= (heap. Body roll does not influence the camber of wheels.

Disadvantage= -on0independent, bad ride quality, both wheels cambered on bump.

Who use it ome /merican sedans, >ord 5ustang, >alcon, most 7?.

DeDion Axle

/lthough independent suspensions wereinvented decades ago, non0independentsuspensions still dominated the marketuntil the late 89s. The first reason was=cheap. The second reason= it offers quite

good handling despite of poor ride. incethe wheels are rigidly linked by an a&le,they remain perpendicular to the roadsurface regardless of body roll. Thereforethe car corners quite stable. %n contrast,in many types of independentsuspensions, camber angle may bechanged due to body roll.



#owever, as e&plained before, live a&le has too much unsprung weight, thus leads topoor ride quality. Therefore many budget sports cars or coupes chose DeDion /&le *rearsuspensions over live a&le. DeDion a&le suspension has much less unsprung weight because the final drive )differential and driving shafts are not rigidly attached to the wheels. :ike independent

suspensions, they are part of the car body and fle&ibly linked to the wheels by universal ;oints. %n other words, they are sprung. The wheels are interconnected by a DeDion Tube, which has a sliding ;oint to permitwheel track variation during suspension movement, this help refining ride quality too. TheDeDion tube keeps both wheels parellel to each other under all conditions, so they arealways perpendicular to the road surface regardless of body roll.

/dvantage= till cheaper than most independent suspensions. Body roll does notinfluence the camber of wheels. Better ride quality than :ive /&le.

Disadvantage= -on0independent, ride is still worse than independent suspensions. Both

wheels cambered on bump.

Who use it (aterham, ?ector, mart.



Independent

SuspensionsSwing axle

suspension

This is a very old independent suspension, used by some sports cars since the 9s, suchas ?W Beetle, 2orsche 6@ *which was based on Beetle and 5ercedes+ famous 699:Aullwing *!C. #owever, it disappeared for at least ' decades because it has so much

weakness. The only advantage is 0 it provides independent shock absorption. #andling is really awful, as camber angle can be noticeably changed by bouncing motion*as shown in the first picture, change of static weight of the car *second picture andbody roll *third picture. 3specially is the body roll, which makes both wheels leantowards the corner, thus result in severe oversteer. This e&plain why the 5ercedes699: Aullwing was criticised as very unpredictable and difficult to handle. (amber variation can be reduced via using longer swing arms, but this could createproblems in packaging. %t engages the space for rear seats and even the boot.

/nother solution is to introduce inherent understeer by setting the wheels negativecambered. This could compensate the oversteer during cornering but the drawback is theinstability in straight line. To cars as slow as ?W Beetle, swing a&le shows its advantagein ride comfort over contemporary non0independent suspensions while the weakness inhandling is not easily seen. >or 2orsche 6@, at least in the less0powerful early versions,the problem is not severe, too. %n later years, when the car got bigger and bigger engine,2orsche realised that the days for swing a&le had nearly finished. That came true whenthe !! launched in !@6, used trailing arm at the rear instead of swing a&le.



/dvantage= %ndependent ride.

Disadvantage= ?ery bad handling.

Who use it 5ercedes 699: *!C, ?W Beetle, 2orsche 6@ etc.

Doule wis!ones suspensionTo many suspension designers, double wishbones *or "/0arms" is the most ideal suspension. %t can be used in front andrear wheels, it is independent and most important, it has nearperfect camber control. >or C9 years and even today, this is thefirst choice for racing cars, sports cars and demanding sedans.

Basically, double wishbones suspension always maintains thewheel perpendicular to the road surface, irrespective of thewheel+s movement. This ensure good handling.

Traditional double wishbones consists of ' parellel wishbone arms of equal length, whichhas the drawback of e&cessive tire scrubbing because of the large variation in track widthas the wheel moved off the neutral position. Therefore engineers developed une!ual-lengt non0parellel /0arms to solve this. By tilting the upper /0arm, anti0dive function isalso achieved.

"" #orsce $$%&s rear suspension

Double wishbones suspension has been very popular in /merican cars. -ot so in 3urope because cars in there aresmaller thus cannot accommodate this relatively space0

engaging suspension. Besides, it is more costly than5ac2herson strut and torsion beam because it involvesmore components and more suspension pick up points inthe car body. 1wing to these reasons, very few small cars

adopt it. 1ne of the few e&amples is #onda (ivic.

This does not mean /merican cars have better handling.-o, due to their larger si$e and weight and the less effortspent in suspension tuning, the ma;ority of doublewishbones0equipped /merican cars actually handlesworse.

/dvantage= %deal camber control leads to good handling.

Disadvantage= pace engaging and costly.

Who use it /merican sports cars and some sedans, most 3uropean pure sports carslike >errari, T?4, :otus .... some 3uopean sedans, most #onda .... manymany many.



"ac#!erson strut suspension

The 5ac2herson strut suspension was invented in the !C9s by3arl . 5ac2herson of >ord. %t was introduced on the !93nglish >ord and has since become one of the dominating

suspensions systems of the world because of its compactnessand low cost. 7nlike other suspension designs, in 5ac2herson strutsuspension, the telescopic shock absorber also serves as a linkto control the position of the wheel. Therefore it saves the uppercontrol arm. Besides, since the strut is vertically positioned, thewhole suspension is very compact. To front0wheel drive cars,whose engine and transmission are all located inside the frontcompartment, they need front suspensions which engage verylittle width of the car. 7ndoubtedly, 5ac2hersion strut

suspension is the most suitable one. -evertheless, this simple design does not offer very good handling. Body roll and wheel+smovement lead to variation in camber, although not as severe as swing a&le suspension.>rom a designer+s viewpoint, its relatively high overall height requires a higher hood andfender line, which is not very desirable to sports cars+ styling.

"" H'undai Ato(&s )ac#erson strut

:ike double wishbones suspension, 5ac2herson strut can beadopted in both front and rear wheels. %n the 9s, there are manybudget sedans employed 5ac strut in all corners, the most famousis >iat+s Type C and Tipo platforms, on which >iat (roma, :ancia

Thema, aab 999, >iat Tipo, Tempra, :ancia Delta, Dedra etc.were based. -one of them was famous of handling. Basically, /lfa4omeo+s AT? ) pider is also based on the Tipo platform, however,after e&perienced unsatisfatory handling during testing, the rear5ac2herson struts were replaced by the pricier multi0linksuspensions.

/dvantage= (ompact and cheap.

Disadvantage= /verage handling.

Who use it 5ost front0wheel drive compact cars.



$railing arm and Semi-trailing arm

suspension

(ompare with the following rearsuspensions, Trailing arm ) emi0trailingarm suspensions are rather old. %t wascommonly used in nearly all mid0price tohigh0price sedans before multi0link rearsuspension became popular in !9s.>rom +' B5W 60series to 5ercedes@93(, even the 2orsche !!, trailingarm ) semi0trailing arm suspensionsdominated half the world. *railing arm suspension *the upper

picture employs two trailing arms whichare pivoted to the car body at the arm+sfront edge. The arm is relatively largecompare with other suspensions+ controlarms because it is in single piece and theupper surface supports the coil spring. %tis rigidly fi&ed to the wheel at the otherend.

-ote that it only allows the wheel to move up and down to deal with bump. /ny lateralmovement and camber change *with respect to the car body is not allowed.-evertheless, when the car rolls into a corner, the trailing arm rolls for the same degreeas the car body, thus changes camber angle *with respect to the road surface. -ow, you

can see both wheels lean towards the outside of the corner, thus lead to understeer.Because of this reason, pure trailing arm was forgotten by car makers long long ago.%nstead of it, they adopted semi0trailing arm. Semi-trailing arm suspension *the lower picture has the trailing arm pivoted at inclinedangles 0 about 9 to 89 degrees. 1therwise are the same as trailing arm suspension.

/pparently, the semi0trailing arms are half trailing and half transverse. Eou can analyse itby splitting it into two vectors, one is the trailing component and another is the transversecomponent. The trailing component leads to understeer, as already mentioned. 1n theother hand, the transverse component is actually equals to a swing a&le suspension.-ow, you may remember that the swing a&le suspension always introduce oversteer dueto body roll. /s a result, the two components cancel each other and result in near neutralsteering response. emi0trailing has a disadvantage 0 when the wheel moves up and down, camber anglechanges, unlike double wishbones suspension. -o matter semi0trailing arm or pure trailing arm suspensions, since they are rigidlyattached to the wheels, inevitably more shock and noise could be transferred to the carbody, especially under hard cornering or running on bumpy roads. 5oreover, a lot ofunsprung weight of the trailing arm leads to poorer ride quality. Therefore most modernsedans replace it with multi0link or double wishbones suspension. Trailing arm ) emi0



trailing is disappearing in the industry.

/dvantage= /ll round, few weakness.

Disadvantage= :ess refined than multi0link.

Who use it 5any sedans and coupes.

$orsion eam suspension

5ost modern mini cars up to (0segment *for instance, ?W Aolf employ torsion beam asthe rear suspension. Why compare with double wishbones, multi0link and trailing armsuspensions, it engages little width of the car, thus enable greater rear seat room. %t ischeaper too. (ompare with 5ac2herson strut, its shock absorber is shorter and can beinclined steeply away from the vertical, thus engage less boot space. %n fact, torsion beam suspension is only half0independent 0 there is a torsion beamconnecting both wheels together, which allows limited degree of freedom when forced.>or some less demanding compact cars, this save the anti0roll bars. 1n the contrary, itdoesn+t provide the same level of ride and handling as double wishbones or multi0linksuspensions, although in reality it is superior to its only direct competitor, 5ac2hersonstrut. 5ost of the 3urope+s best handling AT%s employed this suspenion.

Gol+&s torsion beam rear suspension

,iat #unto&s torion beam

/dvantage= (ompact, cheap.

Disadvantage= Theoretically inferior ride and handling.

Who use it 5ost 3uropean mini cars up to Aolf0class.

"ulti-lin% suspension



ince the late 9s, multi0link rear suspension is increasingly used in modern sedans andcoupes. The earliest applicants include -issan '99F, %nfiniti GC, 5ercedes 0classand B5W 60eries etc. %t is difficult to describe its construction because it is not strictly defined. %n theory, anyindependent suspensions having 6 control arms or more are multi0link. Different designs

may have very different geometry and characteristic, for e&ample, B5W+s multilink lookslike a letter "H", thus gave its name "H0a&le". %t is relatively space0engaging but offers verygood handlingI #onda /ccord+s multi0link is essentially a double wishbones suspensionadded with the fifth control arm. /udi /C+s Guadralink front suspension has four links. %tlooks alike double wishbones but eliminates torque steer.

"" Honda Accord&s -lin. rear suspension%t is too early to say whether multi0link suspension offer handling on a par with doublewishbones. 5ost sports cars and all the best racing cars still use double wishbones. 1nly2orsche 6 and @, -issan kyline AT04 etc. chose multi0link instead. #owever, itseems that multi0link can offer better compromise between handling and space efficiency,as more and more sedans adopt it. #onda, which used to be a loyal supporter of doublewishbones, shifted to multi0link setup in the latest /ccord could be an evidence.

/dvantage= Aood handling and ride.

Disadvantage= -ot as cheap and as compact as 5ac2herson and Torsion beam.

Who use it 5id0si$e to lu&urious sedans.

&eissac! axle suspension

%n the mid089s, 2orsche developed this unique rear suspension for its award0winning'. Basically it is a variant of semi0trailing arm suspension. %n any suspensions, the pivot ;oints must be inserted with rubber bushing to absorb noise

and vibration. >or conventional semi0trailing arm suspension *first row in the followingpicture, whenever under braking, the momentum of the car body tries to pull the car"away" from the rear suspension. Due to the elasticity of the rubber bushing, the rearwheel will toe0out. /s a result, the car will oversteer. Weissach a&le was designed to eliminate this oversteer *second row in the picture. Bysplitting one trailing link into two pieces, with a pivot ;oint added between them, theoversteer under braking can be elinimated or even introduce some understeer J This helpstabli$ing the cornering motion. ome people call it as "passive rear0wheel steering".



Who useit

2orsche '.



$railing arm and Semi-trailing arm

suspension

(ompare with the following rearsuspensions, Trailing arm ) emi0trailingarm suspensions are rather old. %t wascommonly used in nearly all mid0price tohigh0price sedans before multi0link rearsuspension became popular in !9s.>rom +' B5W 60series to 5ercedes@93(, even the 2orsche !!, trailingarm ) semi0trailing arm suspensionsdominated half the world. *railing arm suspension *the upper

picture employs two trailing arms whichare pivoted to the car body at the arm+sfront edge. The arm is relatively largecompare with other suspensions+ controlarms because it is in single piece and theupper surface supports the coil spring. %tis rigidly fi&ed to the wheel at the otherend.

-ote that it only allows the wheel to move up and down to deal with bump. /ny lateralmovement and camber change *with respect to the car body is not allowed.-evertheless, when the car rolls into a corner, the trailing arm rolls for the same degreeas the car body, thus changes camber angle *with respect to the road surface. -ow, you

can see both wheels lean towards the outside of the corner, thus lead to understeer.Because of this reason, pure trailing arm was forgotten by car makers long long ago.%nstead of it, they adopted semi0trailing arm. Semi-trailing arm suspension *the lower picture has the trailing arm pivoted at inclinedangles 0 about 9 to 89 degrees. 1therwise are the same as trailing arm suspension.

/pparently, the semi0trailing arms are half trailing and half transverse. Eou can analyse itby splitting it into two vectors, one is the trailing component and another is the transversecomponent. The trailing component leads to understeer, as already mentioned. 1n theother hand, the transverse component is actually equals to a swing a&le suspension.-ow, you may remember that the swing a&le suspension always introduce oversteer dueto body roll. /s a result, the two components cancel each other and result in near neutralsteering response. emi0trailing has a disadvantage 0 when the wheel moves up and down, camber anglechanges, unlike double wishbones suspension. -o matter semi0trailing arm or pure trailing arm suspensions, since they are rigidlyattached to the wheels, inevitably more shock and noise could be transferred to the carbody, especially under hard cornering or running on bumpy roads. 5oreover, a lot ofunsprung weight of the trailing arm leads to poorer ride quality. Therefore most modernsedans replace it with multi0link or double wishbones suspension. Trailing arm ) emi0



trailing is disappearing in the industry.

/dvantage= /ll round, few weakness.

Disadvantage= :ess refined than multi0link.

Who use it 5any sedans and coupes.

46386926 suspension geometry

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